Many electrical circuits emit electromagnetic radiation when in operation. Electromagnetic interference (EMI) codes established by various regulatory bodies impose limitations upon acceptable levels of electromagnetic radiation that may be emitted by electrical apparatus, such as a test and measurement instrument, in order to prevent interference with other equipment. Frequently, it is not practicable to design the circuits of a test and measurement instrument so that the level of electromagnetic radiation emitted by the circuits is below the levels of the EMI codes, and therefore it is necessary to mount the circuits in an enclosure that contains the radiation emitted by the circuits and shields other equipment from electromagnetic radiation emitted by the instrument circuits. It is also desirable that ambient radiation be prevented from entering the test and measurement instrument and affecting the operation of its circuits.
A detail of the enclosure of a conventional test and measurement instrument, specifically an oscilloscope, is shown in FIG. 1 of the accompanying drawings. The instrument enclosure comprises a main chassis 2 from which the functional elements of the instrument, e.g., circuit boards and a CRT, are supported. The main chassis is made of metal, and includes a front plate 4, a back plate 24, and side rails 12 that join the front and back plates. Flanges 14 extend somewhat towards the back of the instrument from the front plate 4. The main chassis is made by stamping openings in a rectangular sheet of metal and then bending parts of the sheet to form the distinct front and back plates, the side rails and various flanges, including the flanges 14. Overlying the front plate 4 of the chassis is an instrument front panel 6. The panel 6 is made of synthetic plastic material and bears indicia relating to the functions and operation of the instrument. The front panel is secured to the chassis by means of a frame 10 which also is made of synthetic plastic material. The main chassis, with the front panel 6 and the frame 10 attached thereto, is slid into a cabinet 20, and the main chassis is secured to the cabinet using screws (not shown) that extend through the back wall 22 of the cabinet and engage the back plate 24 of the chassis. The cabinet 20 has a rim 26 that is received in the space 18 between the flanges 14 and the frame 10.
In order to meet current EMI codes, and also to provide a convenient path for ground current, it is necessary that the cabinet 20 and the chassis 2 together provide a substantially closed electrically conductive surface enclosing the electrical components of the instrument, and that the maximum linear dimension of any discontinuity in the electrically conductive surface be less than about 4 cm in length. It is therefore important that the length of any electrical discontinuity between the cabinet and the chassis in the region of the rim 26 be less than about 4 cm in length. Of course, the maximum permissible length of a discontinuity depends on the frequency of the radiation that is to be contained, and therefore it may, in some cases, be greater than 4 cm and in others it may be less than 4 cm.
In order to minimize discontinuities between the cabinet 20 and the chassis 2 in the region of the rim 26 of the cabinet, a flexible, electrically conductive gasket is placed in the space 18 between the flange 14 and the frame 10, ahead of the rim 26 of the cabinet. The gasket comprises a core 30 of flexible synthetic plastic material and a sleeve 32 of conductive mesh through which the core 30 extends. When the rim of the cabinet is pushed into the space 18, the gasket is compressed between the rim 26 of the cabinet and the frame 10 and is distorted in shape so that it makes contact with the front plate 4 or the flange 14.
The forces exerted on the gasket are such as to ensure a firm pressure contact between the gasket and both the rim 26 and the front portion of the frame 10, but since the frame 10 is made of dielectric material this has substantially no shielding effect with respect to electromagnetic radiation emitted by the electrical components of the oscilloscope. The nature of the gasket is not such as to ensure that an electrically-conductive pressure contact between the gasket and the chassis will be reliably achieved with discontinuities less than 4 cm in length.